1. Field of the invention
This invention relates in general to flight control devices for airplanes, and in particular to a device to produce directional moments to control yaw of the airplane while under high angle of attack.
2. Description of the Prior Art
Highly maneuverable airplanes are traditionally required to operate in flight regions where the angle of attack, also called "incidence angle", is large. The angle of attack is approximately the angle between the longitudinal axis of the airplane and the direction of its motion. For the airplane to be able to fly safely and perform maneuvers at a high angle of attack, it must be stable and controllable.
Modern airplanes are not required to be inherently stable, but must appear to be so to the pilot. Stability is the tendency of the airplane to return to its initial state when subjected to a disturbance. To achieve apparent stability with an inherently unstable airplane requires that the flight conditions of the airplane be measured and fed back into a flight control computer. The computer then commands the flight controls to generate forces and moments that arrest unwanted airplane motions. Examples of the airplane flight conditions include its attitude with respect to the airstream, velocity, and rotational motions. If it is desired to maneuver the airplane, it must generate forces and moments to reorient itself or change its direction of motion. Both artificial stabilization and maneuvering require substantial forces and moments to be generated by the controls, particularly in the high incidence, or high angle of attack flight region.
To artificially stabilize and control the airplane, control forces and moments, particularly rotational moments, must be available about all primary axes of the aircraft. These axes are the pitch, or nose up, nose down axis; the yaw, or nose right, nose left axis; and the roll, or wing up, wing down axis. The invention described herein is concerned particularly with the yaw axis. At large aircraft incidence angles, natural motions of the aircraft about this axis tend to be unstable unless artificially controlled. Furthermore, standard maneuvers to change the flight path, or direction of motion, of the aircraft require that the aircraft be rotated about an axis that is coincident with the direction of the aircraft motion. Both of these needs dictate that the aircraft possess the capability to generate large control moments about the aircraft yaw axis.
Due to the nature of the flow about aircraft, traditional means of yaw control are ineffective in the high incidence flight region. This is due to the fact that aerodynamic controls usually operate on the principle of generating lift. Lift is a force acting generally normal to the flight path. At high aircraft incidence angles, the air flow over the surface separates from it and destroys the lifting forces. If an aerodynamic component is located at the rear of the aircraft, it is often blocked by portions of the aircraft ahead of it during flight at high angles of attack. It is then in a region of airflow that is so disturbed that it cannot effectively generate forces, even if its own incidence is relatively small.
Up until recent years, even high performance aircraft were not intentionally operated at incidence angles larger than those at which the maximum lift
force was developed. The primary means of yaw directional control was the rudder, which is the hinged control surface attached to the end of the vertical tail or tails. The major problem with the rudder is that its effectiveness is greatly diminished at large aircraft incidence angles, due to the blockage of the wings and forward fuselage, and hence its immersion in a region of highly disturbed flow. Because of these problems, it is not suitable as a yaw control device at very high aircraft incidence angles.
Another means of directional control is differential deflection of wing controls or of the horizontal tails. In this case, the flaps or tail panels are deflected to different settings on opposite sides of the aircraft. While the primary intent of this method is to generate differential lift, or upward force, for rolling the aircraft about its longitudinal axis, yawing moments are also produced. This is due to the differences in drag, or longitudinal forces, on opposite sides of the aircraft as well as induced effects on the vertical tail surfaces. This differential drag produces a yawing moment on the aircraft. While this control moment will work to fairly large aircraft incidence angles, the total moments available are relatively small. In the case of differential horizontal tail deflection, use of these surfaces to generate yaw moments reduces the amount of moment available in the pitch axis, which is the primary axis of operation of the horizontal tail. Such a loss in pitch control can greatly impact the ability of the aircraft to operate at large incidence angles and can result in loss of control during maneuvering flight.
A more recent means of yaw control employs strategically placed holes or slots on the forebody of the aircraft. When control moments are required, air is injected through these holes or slots at a predetermined rate and direction. This additional air modifies the normal flow pattern over the aircraft forebody, and relatively large yawing moments can be generated, such moments being available at large aircraft incidence angles. While effective, this method of yaw control has many disadvantages. It requires that pneumatic lines be run through the forward section of the aircraft. Typically, these will have to be routed very close to the forward part of the nose. The presence of these pipes and tubes ahead of what is the normal location of the radar transmitting antenna can dramatically reduce the performance of any radar system carried. Such a pneumatic control system also requires a source of relatively high pressure air. This air must be taken from the engine or stored in tanks. High pressure air taken from the engine will reduce its overall performance and the thrust available to propel the aircraft. Finding a position for storage tanks of adequate size may be difficult, and such tanks will add weight to the aircraft, which will also tend to reduce its overall performance.
Another method developed recently to generate moments for yaw control also makes use of forces generated on the forebody of the aircraft. It is also most effective for large aircraft incidence angles. It consists of large movable flaplike devices or panels mounted on the nose of the aircraft. Under conditions when no control moments are required, these panels are retracted so as to match the normal contours of the forebody. When a control moment is required, one of these large panels is moved away from the forebody on only one side of the aircraft. The deflection of this panel alters the flow field on the forebody of the aircraft and causes a directional moment to be generated. The strength and direction of this moment is dependent on the location of the panel, its deflection, and the attitude of the aircraft with respect to the direction of its motion.
While this method of directional control may be effective, it also has many disadvantages. The panels are large, on the order of several feet in length and close to a foot in width. This large size impacts high performance aircraft in several ways. First, the required size and location reduces the volume available in the nose of the aircraft for a radar system. The blockage caused by these panels and their associated hardware can be assumed to reduce the performance of the radar system that is fitted in this area. Since the size of the equipment needed to move the panels is related to the size of the panels themselves, these actuation devices may be large and heavy. Since only one of the control panels is moved at any one time, it may be necessary to have two actuation devices. In summary, while this means of directional control for high incidence may be effective, it is heavy, bulky, and requires a complex actuation system. All of these would tend to reduce the overall performance of the aircraft.